Tamiya XR311 Project

Page 1:  Building the Chassis

   

The XR311 comes in a large box with lovely artwork.  Note that this kit bears the same model number as the original so a special sticker has been added to indicate that this is a re-release.  Inside the sections are nicely divided and the metal parts are segregated into a smaller inner box with dividers to keep them safe.


   

The left hand image shows the contents of the inner box for the metal parts.  This includes the chassis tray, the electronics sponsons and the gearbox housing.  The right hand image shows the fairly small number of parts for this vehicle.  At the time it was made, the chassis and suspension were very complex but it seems somewhat simple in hindsight.




The first step assembles a bunch of metal components which comprise the bulk of the chassis frame.  This picture shows the bottom of the frame.  The bracket you see toward the front with the cam shaped slots is used to lock on the body.  The black plastic parts are the splined supports for the torsion bars.  The rear metal parts are electronic trays.  Much of this is the same as the original, but the shape of the electronic trays has been updated.  The original were flat (and therefore weak).  New electronics are smaller so we can get away with tubbed trays.


   

The suspension build starts at the front.  The kit comes with metal bushings throughout.  Although I always like to replace these with ball bearings and I bought a bearing kit from Fast Eddy for this purpose, the bearing kit did not include flanged bearings so I still needed to use the kit provided bushings for the front axle.  Normally the axle is fixed and the bearings rotate with the wheel, but in this case the whole front axle rotates because it bolts directly to the wheel.  The front C-hubs are actually built up from 3 parts as shown.  There is an upper and lower arm with a standoff between.  These clamp around the plastic kingpin of the steering knuckle.  This is identical to how it was done in the original model 40 years ago.


   

Each upper and lower suspension arm comes in two pieces and clamps around the C-hub as shown.  This setup makes for a lot of little fiddly parts, especially given that this is a 1/12th scale model.  The steering method is fascinating.  What you see here is the aft bulkhead of the front suspension which carries a pair of tiny bellcranks.  The cross bar you see between them has a small stud protruding from the top.  That stud will push on a slot in a C-shaped clip to move the steering.  The flexibility of the clip acts as a servo saver.


   

The left hand image shows the front suspension assembly viewed from the front.  The steering tie rods are just wires with shepherd hooks on the ends as shown.  There is therefore no toe adjustability.  The right hand image shows the same assembly from the top.  You can see the two tiny bellcranks and the steering cross bar.  The cross wrench has been added to the picture for scale to show you how small this suspension is.


   

After the addition of a few metal standoffs and a front and rear support bracket, the front suspension assembly is complete.  It is actually quite complicated.  The second image shows the vertical crank used for steering input.


   

Now the steel plate torsion bars can be installed.  These replace the notoriously fragile plastic bars of the original.  The torsion bars slide into spring loaded supports on the chassis, and into slots on the lower suspension arms at the other end.  As the suspension compresses, the lower arms twists the bar and that twist is resisted at the other end.  By compressing the spring you can pull the restraint out of the splined socket and rotate the restraint by one tooth to add or subtract preload.  This allows you to tune the suspension to vehicle weight.  It works really well and allows you to get a very realistic suspension motion and stiffness.


   

The rear suspension is a bit simpler.  The vertical hub is one part with an axle inserted.  I was able to use ball bearings here.  Like the front, the upper and lower suspension arms are two pieces each.  The wishbones are very wide and therefore provide a lot of support for off road use.


   

There is no differential on this vehicle.  The ring gear is solidly connected to a drive cup at either side and fit into a cast gearbox housing.  Both ring and pinion are metal.  In fact, everything in these pictures is metal.  The original used plastic drive cups.


   

The pinion bevel gear is inserted though the rear gearbox housing because the motor will drive from the back.  This is then bolted to the main metal housing as shown.  You can see another tiny flanged bushing here that I wasn't able to replace with a bearing.


   

The suspension arms are sandwiched between the front and rear covers of the gearbox housing.  Before the front cover is installed, you can see the ring gear inside as shown. The bolts used to connect the whole thing together are the longest 3mm screws I've seen so far (55mm).


   

The motor is cradled directly above the rear axle and faces aft.  The kit comes with 3 pinion gears which allow 3 different speeds.  They are labeled A, B, and C which are slow to fast, respectively.  Note that the number of teeth on the outside of the pinion are not the only thing to change.  The integral drive gear at the center also varies.  This means that both gear stages are variable which drives a pretty big difference in available ratios.  Let's take a look assuming a 15 tooth motor pinion and a 40 tooth output drive gear.  The number of teeth is never shown in the instructions so I had to count them manually.  Hopefully I got it right.

So this shows you that there is a 2.4x difference between the high and low gearing options.  The mesh is not adjustable.  Instead, each gear size fits into a different axle support hole in the gearbox housing.  I'm using a standard silver can 540 motor which is more than adequate for a model of this size.  You wouldn't want anything more powerful if you want to keep the model intact.


   

I started by assembling the gearbox with the A gear which is the slowest speed (highest gear reduction).  The motor pinion is aluminum but the A gear and the drive gear are plastic.  As previously mentioned, the ring and pinion of the locked axle are metal.  Once the plastic cover is installed as shown on the right, you can see the 3 round blisters which represent the bearing supports for the 3 different gear sets.  They are arranged in an arc with the motor pinion so they will all mesh without any adjustment.  It is a clever and reasonably fool proof system.


   

The rear torsion bars are installed just like the front but must be preloaded quite a bit more because of the weight of the motor in the back.  Don't forget to install the battery before adjusting the preload because it is a huge component of the overall weight.  The wheels are a 3 piece design which must be assembled directly to the axle.  There is no way to remove the wheel without also taking it apart.  It is not exactly a beadlock since it doesn't clamp tight on the bead, but it does provide good support and, most importantly, eliminates the need for tire glue.  I love the profile and tread pattern on these tires.


    

Here's the completed rolling chassis.  Note that everything between the axles is metal.  The right hand image shows the installation of the durable front bumper.  There's really not that much going on here mechanically or structurally, but the model is very sturdy.  The same can't be said for the body.


   

The original needed to make room for both a steering and a throttle servo as well as the associated MSC, but we have more space to play with here.    The steering servo attaches up front with servo tape and is connected to the steering crank with a short rod.  The rest of the main tub leaves room for a standard 6-cell battery pack.  The trays to either side have plenty of room for an ESC and a receiver.  The last image shows the completed chassis from below.  Barely visible is the clear under tray which protects the torsion bars from obstacle contact.


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©2019 Eric Albrecht